Magnetic recording medium and method for producing the same

ABSTRACT

It is a magnetic recording medium for which favorable values for all of reproduced output, noise level and error rate can be obtained, and which is also superior in terms of storage stability. A magnetic recording medium ( 10 ), which has a configuration in which a magnetic layer ( 3 ) comprised of a maghemite thin film containing cobalt is formed on a tape-shaped substrate ( 1 ) comprised of aromatic polyimide, and in which the substrate ( 1 ) is 3 to 10 μm in thickness, and 5 to 30 nm in its ten-point average roughness (SRz) of the surface on which the magnetic layer ( 3 ) is formed, and the magnetic layer ( 3 ) is 10 to 50 nm in thickness, 0.1 to 3.0 nm in center plane average thickness (SRa), and 2,000 to 4,000 Oe (158,000 to 316,000 A/m) in in-plane coercivity, is made.

TECHNICAL FIELD

[0001] The present invention relates to a high density magneticrecording medium, and particularly to a magnetic recording medium thathas favorable storage stability in a system using a magnetoresistivemagnetic head (MR head).

BACKGROUND ART

[0002] In recent years, with respect to magnetic recording media, higherrecording densities are being increasingly demanded. Especially thesedays, in order to achieve further high-density recording, with respectto magnetic heads used in performing reproduction of recorded signals,magnetoresistive magnetic heads (MR heads) are becoming more popular inplace of conventional inductive heads, and are being adopted forso-called magnetic tapes and not only hard disks.

[0003] As recording media for performing recording and reproduction bysuch a magnetoresistive magnetic head (MR head), so-called metal thinfilm type magnetic recording media are used, and in line with the demandfor higher density recording described above, achieving a furtherincrease in coercivity and reduction in noise is becoming necessary.

[0004] In addition, along with the advances in high density recording,the demand for so-called archivability, where storage over extendedperiods without the occurrence of degradation in signal quality ispossible, is becoming more stringent.

[0005] As one that meets such various demands made of magnetic recordingmedia, in Japanese Patent Application Publication No. Hei-11-110731 andJapanese Patent Application Publication No. 2001-250216, there isproposed a magnetic recording medium for use as a hard disk of aconfiguration in which the magnetic layer is formed of a maghemite thinfilm containing cobalt.

[0006] The magnetic recording medium proposed in each of thepublications above, which has a magnetic layer of a ferrite film, hassuch advantages as being more stable, corrosion resistant and superiorin long-term storability as compared to coated type magnetic recordingmedia in which the magnetic layer is formed of a magnetic coating inwhich magnetic powder is dispersed in a binder, or Co/CoO evaporatedmagnetic recording media and the like.

[0007] However, in applying such a magnetic layer of a ferrite film asdescribed above to magnetic tapes, not only must magnetic propertiessuch as the coercivity Hc of the magnetic layer be improved, but theabrasion of the surface due to contact between an MR head and themagnetic tape becomes a problem, and therefore it becomes important tocontrol the surface properties of the magnetic recording medium.

[0008] As such, in the present invention, in view of the problemsdescribed above, surface properties of a magnetic tape type magneticrecording medium in particular are studied, and a magnetic recordingmedium having an appropriate roughness, and in which various propertiessuch as chemical stability, magnetic properties, running stability,running durability are improved, is provided.

DISCLOSURE OF THE INVENTION

[0009] A magnetic recording medium of the present invention is taken tohave a configuration in which a magnetic layer made of a maghemite thinfilm containing cobalt is formed on a tape-shaped substrate made ofaromatic polyimide. It is defined that the thickness of the substrate is3 to 10 μm, the ten-point average roughness (SRz) of the surface onwhich the magnetic layer is formed is 5 to 30 nm, the thickness of themagnetic layer is 10 to 50 nm, the center plane average roughness (SRa)is 0.1 to 3.0 nm, and the in-plane coercivity is 2,000 to 4,000 Oe(158,000 to 316,000 A/m).

[0010] It is defined that in a method of manufacturing a magneticrecording medium of the present invention, a foundation layer and amagnetic layer, which is made of a maghemite thin film containingcobalt, are formed sequentially on a tape-shaped substrate made ofaromatic polyimide and whose thickness as supported while running is 3to 10 μm, and whose ten-point average roughness (SRz) of the surface onwhich the magnetic layer is formed is 5 to 30 nm. It is defined that thethickness of the magnetic layer is 10 to 50 nm, the center plane averageroughness (SRa) is 0.1 to 3.0 nm, and the in-plane coercivity is 2,000to 4,000 Oe (158,000 to 316,000 A/m).

[0011] According to the present invention, favorable values can beachieved for all of reproduced output, noise level and error rate, and amagnetic recording medium that is by far superior in terms of storagestability as compared to existing magnetic tapes can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic sectional view of a magnetic recordingmedium of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

[0013] Specific embodiments of a magnetic recording medium of thepresent invention will be described, however the present invention isnot to be limited to the examples below.

[0014] A schematic sectional view of an example of a magnetic recordingmedium 10 of the present invention is shown in FIG. 1.

[0015] The magnetic recording medium 10 has a configuration in which afoundation layer 2, a magnetic layer 3 and a protective layer 4 areformed sequentially on a tape-shaped non-magnetic substrate 1.

[0016] The substrate 1 is formed of, in particular, aromatic polyimide.

[0017] For the substrate 1, considering use as a magnetic tape, onewhich is tape-shaped and whose thickness is 10 μm or below is suitable.In addition, in order to secure sufficient contact with the magnetichead, it is necessary that it be 3 μm or above.

[0018] The substrate 1 may have a single layer structure or amulti-layer structure in which two or more layers are layered. Further,some adhesive layer may be provided on the surface of the substrate 1 soas to enhance adhesion with the later-described foundation layer 2 orback coat layer 6.

[0019] In order to improve the durability and running characteristics ofthe magnetic recording medium 10 ultimately obtained, as well ashandling during film formation of the magnetic tape, fine particles maybe provided internal to the aromatic polyimide substrate 1. As thesefine particles, for example, calcium carbonate, silica, alumina and thelike may be used.

[0020] In addition, in order to prevent the occurrence of scraping dueto contact between the magnetic recording medium 10 and a magnetic head,the ten-point average roughness (SRz) of the side of the aromaticpolyimide substrate 1 on which the magnetic layer is formed is made tobe 5 nm or greater. However, when SRz exceeds 30 nm, since the spacingbetween the magnetic head and the magnetic recording medium 10increases, recording and reproducing characteristics degrade, and errorrate increases, it is preferable that the ten-point average roughness(SRz) of the side of the aromatic polyimide substrate 1 on which themagnetic layer is formed be 5 nm to 30 nm.

[0021] The foundation layer 2 can be formed by depositing a NiO film onthe substrate 1 by sputtering a Ni metal as a target under an oxygen andargon atmosphere through, for example, a reactive sputtering method.

[0022] The magnetic layer 3 can be formed by, after the foundation layer2 is formed as described above, sputtering a magnetic metal alloycontaining Co as a target under an atmosphere of oxygen and argon toform a magnetite film containing Co and then performing a thermaltreatment to make it a maghemite film containing Co.

[0023] If the magnetic layer 3 is formed with, for example, an alloy ofFe and Co as a target, it is preferable for the amount of Co added inthe maghemite film containing Co be 1 to 10 weight % relative to Fe.Although the coercivity of the magnetic recording medium 10 can be madehigher by increasing the amount of Co added, when the amount of Co addedexceeds 10 weight %, the stability of the magnetic recording medium 10over time decreases.

[0024] It is preferable that the in-plane coercivity of the maghemitefilm containing Co constituting the magnetic layer 3 be 2,000 to 4,000Oe (158,000 to 316,000 A/m). This is because in order to realize lownoise and high resolution for the magnetic recording medium, it isnecessary that it be 2,000 Oe (158,000 A/m) or greater, and on the otherhand because when the in-plane coercivity exceeds 4,000 Oe (316,000A/m), sufficient recording becomes impossible and reproduced outputdecreases.

[0025] 10 to 50 nm is suitable as the thickness of the magnetic layer 3.This is because when the thickness of the magnetic layer is made lessthan 10 nm, sometimes sufficient coercivity cannot be obtained, and onthe other hand, because when it is formed thicker than 50 nm, coercivitybecomes higher, but the particles forming the film become larger, as aresult of which noise increases and further a problem arises in that MRheads saturate, and distortion occurs in the reproduced output.

[0026] In addition, the center plane average roughness (SRa) of themagnetic layer 3 is made 0.1 to 3.0 nm. When the center plane averageroughness (SRa) is made less than 0.1 nm and the surface of the magneticlayer 3 is made smooth, it causes an increase in error rate due tooccurrences of scraping due to friction between the magnetic recordingmedium 10 and magnetic heads, and there is also a problem in that wearof magnetic heads increases. On the other hand, when the surface of themagnetic layer 3 is made rougher such that the center plane averageroughness (SRa) exceeds 3.0 nm, there is a problem in that the spacingbetween the magnetic recording medium and magnetic heads increases,sufficient recording and reproducing properties cannot be achieved, anderror rate increases.

[0027] The protective layer 4 may be formed through a PVD method such assputtering and the like or through a CVD method.

[0028] In forming it through a CVD method, a hydrogen carbonate gas or amixed gas of hydrogen carbonate gas and inert gas is introduced into avacuum vessel, and plasma of hydrogen carbonate gas is generated bydischarging electricity in the vacuum vessel under a condition in whichpressure is held between 10 to 100 [Pa] to form a thin film of carbon onthe magnetic layer 3. For discharging electricity, either of an externalelectrode method and an internal electrode method is good, and thedischarge frequency is determined experimentally.

[0029] In addition, by applying a voltage of 0 to −3 [kV] to theelectrode on the substrate side, it is possible to enhance the hardnessand improve the adhesiveness of the carbon protective layer 4.

[0030] As the hydrogen carbonate gas mentioned above, methane, ethane,propane, butane, pentane, hexane, heptane, octane, ethylene, acetylene,propene, butene, pentene, benzene and the like may be applied.

[0031] It is preferable that the protective layer 4 be formed in athickness of 6 to 10 nm so that it does not affect the spacing of themagnetic recording medium 10 or its properties as a magnetic tape.

[0032] On the protective layer 4 mentioned above, in order to improverunning characteristics, an optional perfluoropolyether lubricant may beapplied to form a lubricant layer 5.

[0033] In addition, an optional back coat layer 6 may be formed on theside opposite the side on which the magnetic layer 3 is formed.

[0034] It is preferable that the electric resistance of the surface ofsuch a magnetic recording medium 10 as described above be 50 to 3,000MΩ, and it is further preferable that it be 0.1 to 10 MΩ.

[0035] The electric resistance of the surface of the magnetic recordingmedium 10 can be controlled to a desired value by adjusting the amountof oxygen supplied during sputtering at the time of forming the magneticlayer 3, or by forming an optional foundation layer using Cu, Cr, Al orthe like.

[0036] Hereinafter, specific examples of a magnetic recording medium ofthe present invention will be described based on experiment results.

EXAMPLE 1

[0037] First., the foundation layer 2 is formed on the substrate 1.

[0038] As the substrate 1, a continuous polyimide film of a thickness of7 μm was prepared, and was contacted with the peripheral surface of acan roll, which was equipped with heating means, to be heated to 150° C.Further, it was carried at a speed of 10 m/min, and a NiO film of athickness of 50 nm was formed by sputtering a Ni metal target by areactive sputtering method in an atmosphere comprised of argon andoxygen where the oxygen partial pressure was 0.1 Pa, and the totalpressure was 0.4 Pa.

[0039] Next, the magnetic layer 3 is formed on the foundation layer 2.

[0040] A magnetite film containing Co was formed on the substrate 1, onwhich the foundation layer 2 (the NiO film) was formed as describedabove, in a thickness of 30 nm by performing sputtering with a Fe+4 (wt%) Co metal alloy as a target under a thermal condition of 150° C. andin an atmosphere comprised of argon and oxygen where the oxygen partialpressure was 0.07 Pa and the total pressure was 0.5 Pa.

[0041] Next, the magnetic layer 3 comprised of a maghemite film wasformed by carrying, in an oven heated to 300° C. in the atmosphere, thesubstrate 1 on which the magnetite film was formed as mentioned above ata feeding speed of 5 m/min, and performing a thermal treatment.

[0042] Next, the protective layer 4 comprised of a diamond-like carbonfilm was formed on the magnetic layer formed as described above, thelubricant layer 5 was formed by applying a perfluoropolyether lubricanton the protective layer 4, and the desired magnetic recording medium 10was made.

[0043] Further, this was cut in widths of 8 mm and sample magnetic tapeswere obtained.

[0044] Next, of the conditions for making the magnetic recording medium,by altering predetermined conditions as mentioned below, and keepingother manufacturing conditions similar to example 1 above, samplemagnetic tapes of example 2 through example 9 and comparative example 1through comparative example 5 were made.

[0045] In addition, as comparative examples 6 and 7, commercially soldmagnetic tapes were adopted.

EXAMPLE 2

[0046] The thickness of the substrate 1 comprised of aromatic polyimidewas made 3 μm.

EXAMPLE 3

[0047] The ten-point average roughness (SRz) of the side of thesubstrate 1 comprised of aromatic polyimide on which the magnetic layeris formed was made to be 30 nm.

EXAMPLE 4

[0048] The thickness of the magnetic layer 3 comprised of a cobaltferrite thin film was made to be 10 nm.

EXAMPLE 5

[0049] The thickness of the magnetic layer 3 comprised of a cobaltferrite thin film was made to be 50 nm.

EXAMPLE 6

[0050] The coercivity of the magnetic layer 3 comprised of a cobaltferrite thin film was made to be 2,000 Oe (158,000 A/m).

EXAMPLE 7

[0051] The coercivity of the magnetic layer 3 comprised of a cobaltferrite thin film was made to be 4,000 Oe (316,000 A/m).

EXAMPLE 8

[0052] The gas pressure at the time of forming the magnetic layer 3comprised of a cobalt ferrite thin film was made to be 0.6 Pa, and thecenter plane average roughness (SRa) of the magnetic layer 3 was made tobe 3.0 nm.

EXAMPLE 9

[0053] In place of the heating treatment in the atmosphere in example 1above, a sputtering process was performed under an atmosphere havingexcessive oxygen, and thus a magnetic layer 3 comprised of a maghemitefilm whose center plane average roughness (SRa) is 0.6 nm was formed.

COMPARATIVE EXAMPLE 1

[0054] As the substrate 1, an aromatic polyimide film of a thickness of2 μm was adopted.

COMPARATIVE EXAMPLE 2

[0055] The ten-point average roughness (SRz) of the substrate 1comprised of an aromatic polyimide film on the side on which themagnetic layer is formed was made to be 4 nm.

COMPARATIVE EXAMPLE 3

[0056] The thickness of the magnetic layer 3 comprised of a cobaltferrite thin film was made to be 8 nm.

COMPARATIVE EXAMPLE 4

[0057] By an increase in the amount of cobalt contained in the FeCotarget at the time of forming the magnetic layer 3, the coercivity ofthe magnetic layer 3 comprised of a cobalt ferrite thin film was made tobe 5,000 Oe (395,000 A/m).

COMPARATIVE EXAMPLE 5

[0058] The thickness of the magnetic layer 3 comprised of a cobaltferrite thin film was made to be 60 μm.

COMPARATIVE EXAMPLE 6

[0059] For purposes of comparing storage stability, a commercially soldAIT-2 evaporated tape, SDX2-50C, was used.

COMPARATIVE EXAMPLE 7

[0060] For purposes of comparing storage stability, a commercially soldDDS4 evaporated tape, DGD150P, was used.

[0061] In addition, it is assumed that the evaporated tapes ofcomparative examples 6 and 7 mentioned above do not have a protectivelayer nor a lubricant layer formed on a maghemite film containingcobalt.

[0062] With respect to each of the magnetic tapes made as describedabove, electromagnetic conversion characteristics were measured, andevaluations were performed.

[0063] Specifically, after information signals were recorded at arecording wave length of 0.5 μm on each of the sample magnetic tapesusing a modified AIT drive, measurements of each of reproduced output,noise level (the value at a frequency down by 1 MHz from the carriersignal) and error rate were taken with a yoke-type GMR head.

[0064] With respect to the reproduced output, as an evaluationcriterion, the measurement of example 1 was taken to be a referencevalue, and cases which fell below this by 2 dB or more were evaluated asx.

[0065] With respect to noise level, the value of example 1 was taken tobe a reference value, and cases which fell below this by 1 dB or morewere evaluated as x.

[0066] With respect to the error rate, cases that were lower than1.0×10⁻⁴ were evaluated as O, and cases equal to or greater than1.0×10⁻⁴ were evaluated as x.

[0067] As for storage stability, after being left in an atmosphere of40° C. and 80% humidity for a week, those whose saturation magnetizationdropped by 3% or more were evaluated as x, and those whose amount ofdecrease was less than 3% were evaluated as O.

[0068] In addition, the sample magnetic tapes of comparative examples 6and 7 were evaluated only with respect to storage stability.

[0069] The configuration of each sample magnetic tape is indicated intable 1 below, and the evaluation results mentioned above are shown intable 2 below. TABLE 1 Magnetic Layer Substrate (Cobalt Maghemite film)Thickness Thickness (μm) SRz (nm) (nm) SRa (nm) Hc (Oe) Ex. 1 7 15 301.0 2500 Ex. 2 3 15 30 1.0 2500 Ex. 3 7 30 30 1.0 2500 Ex. 4 7 15 10 1.02500 Ex. 5 7 15 50 1.8 2500 Ex. 6 7 15 30 1.0 2000 Ex. 7 7 15 30 1.04000 Ex. 8 7 15 30 3.0 2500 Ex. 9 7 15 30 0.6 2500 Comp. Ex. 1 2 15 301.0 2500 Comp. Ex. 2 7 4 30 1.0 2500 Comp. Ex. 3 7 15 8 0.8 1800 Comp.Ex. 4 7 15 30 1.0 5000 Comp. Ex. 5 7 15 60 2.0 3000 Comp. Ex. 6Commercially sold AIT-2 evaporated tape, SDX2-50C Comp. Ex. 7Commercially sold DDS4 evaporated tape, DGD150P

[0070] TABLE 2 Reproduced Storage Output Noise Level Error RateCharacteristics Ex. 1 ∘ ∘ ∘ ∘ Ex. 2 ∘ ∘ ∘ ∘ Ex. 3 ∘ ∘ ∘ ∘ Ex. 4 ∘ ∘ ∘ ∘Ex. 5 ∘ ∘ ∘ ∘ Ex. 6 ∘ ∘ ∘ ∘ Ex. 7 ∘ ∘ ∘ ∘ Ex. 8 ∘ ∘ ∘ ∘ Ex. 9 ∘ ∘ ∘ ∘Comp. Ex. 1 x ∘ x ∘ Comp. Ex. 2 ∘ ∘ x ∘ Comp. Ex. 3 ∘ x ∘ ∘ Comp. Ex. 4— — — — Comp. Ex. 5 — — — — Comp. Ex. 6 x Comp. Ex. 7 x

[0071] As is apparent from table 1 and table 2 above, with respect tothe magnetic tapes of examples 1 through 9, in which a magnetic layer,comprised of a maghemite thin film containing cobalt and whose thicknessis 10 to 50 nm, center plane average roughness (SRa) is 0.1 to 3.0 nm,and in-plane coercivity is 2,000 to 4,000 Oe (158,000 to 316,000 A/m),is formed on a substrate, comprised of aromatic polyimide and whosethickness is 3 to 10 μm and whose ten-point average roughness (SRz) ofthe side on which the magnetic layer is formed is 5 to 30 nm, favorableresults for all of reproduced output, noise level and error rate, whichare demanded of high density magnetic recording media, were obtained,and with respect to storage stability, too, superior results wereobtained as compared to existing magnetic tapes (comparative examples 6and 7).

[0072] On the other hand, with the magnetic tape of comparative example1, whose thickness of the substrate 1 comprised of aromatic polyimidewas made to be 2 μm, sufficient contact with the magnetic head could notbe achieved, and the reproduced output decreased.

[0073] With the magnetic tape of comparative example 2, whose ten-pointaverage roughness (SRz) of the substrate 1, comprised of an aromaticpolyimide film, on the side on which the magnetic layer is formed wasless than 5 nm, sufficient durability could not be secured, and thereproduced output decreased.

[0074] With the magnetic tape of comparative example 3 whose thicknessof the magnetic layer 3 was less than 10 nm, coercivity was low, and thenoise level increased.

[0075] With the magnetic tape of comparative example 4 in which thecoercivity of the magnetic layer 3 was made to be 5,000 Oe (395,000A/m), distortion occurred in the recorded signals, and reproduced outputcould not be obtained.

[0076] With the magnetic tape of comparative example 5 in which thethickness of the magnetic layer 3 was made to be 60 nm, the reproducingMR head saturated, and reproduction could not be performed.

[0077] According to the present invention, in a magnetic recordingmedium of a configuration in which a magnetic layer comprised of amaghemite thin film containing cobalt is formed on a tape shapedsubstrate comprised of aromatic polyimide, by specifying a configurationin which the thickness of the substrate is 3 to 10 μm, the ten-pointaverage roughness (SRz) of the surface on which the magnetic layer isformed is 5 to 30 nm, the thickness, the center plane average roughness(SRa), and the in-plane coercivity of the magnetic layer are,respectively, 10 to 50 nm, 0.1 to 3.0 nm and 2,000 to 4,000 Oe (158,000to 316,000 A/m), favorable results were obtained for all of reproducedoutput, noise level, and error rate which are demanded of high densitymagnetic recording media, and it was possible to make it one which isalso, by far, superior in terms of storage stability and corrosionresistance as compared to existing magnetic tapes.

1. A magnetic recording medium in which a magnetic layer including amaghemite thin film containing cobalt is formed on a tape-shapedsubstrate including aromatic polyimide, the recording mediumcharacterized in that said substrate is such that thickness is 3 to 10μm and ten-point average roughness (SRz) of a surface on which saidmagnetic layer is formed is 5 to 30 nm, and said magnetic layer is suchthat thickness is 10 to 50 nm, center plane average roughness (SRa) is0.1 to 3.0 nm and in-plane coercivity is 2,000 to 4,000 Oe (158,000 to316,000 A/m).
 2. The magnetic recording medium according to claim 1characterized in that reproduction of recorded signals is carried outusing a magnetic head of a magnetoresistive type.
 3. A method ofmanufacturing a magnetic recording medium, characterized in that asubstrate including aromatic polyimide and whose thickness is 3 to 10 μmand ten-point average roughness (SRz) of a surface on which a magneticlayer is formed is 5 to 30 nm is supported and carried, a foundationlayer, and a magnetic layer including a maghemite thin film containingcobalt are sequentially and continuously formed on said substrate, andthe thickness, center plane average roughness (SRa) and in-planecoercivity of said magnetic layer are made to be, respectively, 10 to 50nm, 0.1 to 3.0 nm and 2,000 to 4,000 Oe (158,000 to 316,000 A/m).